1. Field of the Invention
The present invention relates to an electronic device and a filter that are provided with a balanced terminal.
2. Description of the Related Art
Examples of electronic devices having a balanced terminal are filters and antennas. As a filter having a balanced terminal, there is known for example a band pass filter of unbalanced input/balanced output type. As such a filter, there is one using a balun. The balun is used to perform mutual conversion between an unbalanced signal and a balanced signal. Radio communication equipments such as mobile or cellular phones demand reductions in the dimension and thickness as a filter.
In a line for transmitting an unbalanced signal, a signal is transmitted by the potential of a signal line with respect to a ground potential. In a line for transmitting a balanced signal, a signal is transmitted by the potential difference between a pair of signal lines. A balanced signal is generally considered as being superior in balance characteristics when the phases of signals transmitted between a pair of signal lines are different from each other by 180 degrees, and are of substantially the same amplitude.
FIG. 34 illustrates a general structure of a balun. This balun has a half-wave (λ/2) resonator 101, and first and second quarter-wave resonators 102 and 103. Both ends of the half-wave resonator 101 are open ends, and an unbalanced input terminal 111 is connected to one open end. The short-circuit ends of the first and second quarter-wave resonators 102 and 103 are arranged so as to oppose to the half-wave resonator 101 so that they are opposed to the open ends of the half-wave resonator 101, respectively. Balanced output terminals 112 and 113 are connected to the open ends of the first and second quarter-wave resonators 102 and 103, respectively, thereby forming a pair of balanced output terminals.
As a balun having this structure, there are laminate type balun transformers as described in Japanese Unexamined Patent Publications No. 2002-190413 and No. 2003-007537. Both aim at miniaturization due to a laminate structure that is obtained by forming each resonator with a spiral-like conductor line pattern, and forming the conductor line pattern on a plurality of dielectric substrates. Japanese Unexamined Patent Publication No. 2005-045447 and No. 2005-080248 describe laminate type band pass filters using a half-wave resonator, as a balanced output type band pass filter.
Conventionally, a dipole antenna using a half-wave resonator is known as an antenna that performs a balanced input or a balanced output. This is, as shown in FIG. 35, one in which a pair of balanced terminals 301 and 302 are connected to a half-wave resonator 300, both ends of which are open ends. In the electric field distribution in a basic resonance mode in the open-ended half-wave resonator 300, the electric field is zero at the middle portion in a lengthwise direction, and the maximum at both ends, as shown in FIG. 36. There is a phase reversal of 180 degrees between the right half and the left half from the lengthwise middle portion. Therefore, the input and output of balanced signals can be achieved by connecting the pair of balanced terminals 301 and 302 at a bilaterally symmetrical position where the phase is reversed 180 degrees. There is also known an antenna that performs balanced input and output in combination of a quarter-wave resonator and a balun. Specifically, this antenna performs mutual conversion between an unbalanced signal and a balanced signal by connecting the balun to the quarter-wave resonator provided with an unbalanced terminal, and performs the balanced input and output via the balun. On the other hand, Japanese Unexamined Patent Publication No. 2002-532929 discloses a dipole antenna that performs balanced input and output. This publication also discloses a constructional example that performs balanced input and output through connecting a terminal to each of two pieces of quarter-wave resonators, respectively. In this example, the dimension of the quarter-wave resonator is determined by a quarter-wave of an operating frequency.